In conventional cellular mobile communication systems, a UE (user equipment) has to communicate with the other UE only through the relaying of base stations regardless of the distance between the two UEs. FIG. 1 illustrates the conventional communication mode, where UE1 and UE2 interact through the UTRAN consisting of the base station transceiver (namely Node B) and the Radio Resource controller (RNC), and this communication mode Is also called UP-UTRAN-DOWN mode. However, in some cases when the distance between two UEs who are camping in the same cell is very close, it can be a more reasonable way for them to communicate directly, rather than through the relaying of base stations. This method is the so-called peer-to-peer communication, abbr. as P2P.
FIG. 2 illustrates a P2P communication mode, as FIG. 2 shown, the dashed line represents signaling link, the solid line for data link and the arrowhead for direction of information flow. Only signaling link exists between the UTRAN and the UE, while only data link exists between the two communicating UEs. Assume that only resource for maintaining basic communication is needed. A direct link is taken as one unit of radio resource (with fixed frequency, timeslot and spreading code), it can be easily drawn that P2P communication mode only needs two units of radio resource to maintain basic communication. If additional signaling cost for management is ignored, P2P communication can save about 50% radio resource than conventional communication mode. Meanwhile, the UTRAN still holds control over P2P communication, especially over how to use radio resources, so that network operators can easily charge for the radio resources used by P2P communication.
A method and apparatus for establishing P2P communication in wireless communication networks, as described in the patent application entitled “A Method and Apparatus for Establishing P2P Communication in Wireless Communication Networks”, filed by KONINKLIJKE PHILIPS ELECTRONICS N.V. on Mar. 7, 2003, application Ser. No. 03119892.9, is suitable to any TDD CDMA communication system including TD-SCDMA systems, and incorporated herein as reference.
A method and apparatus for radio link establishment and maintenance with P2P communication in wireless communication networks, as described in the patent application entitled “A Method and Apparatus for Radio Link Establishment and Maintenance with P2P Communication in Wireless Communication Networks”, filed by KONINKLIJKE PHILIPS ELECTRONICS N.V. on Mar. 7, 2003, application Ser. No. 03119895.3, is suitable to any TDD CDMA communication system including TD-SCDMA systems, and incorporated herein by reference.
After establishing uplink synchronization with the UTRAN through the same random access procedure as existing TD-SCDMA systems, the UE can establish a P2P direct link with the other UE, in accordance with the method and apparatus as described in the application document whose application Ser. No. is 03119892.9, i.e.: allocate relevant dedicated resource for two P2P UEs. Then, direct link between the two UEs can be established and maintained in accordance with the method and apparatus as described in the application whose application Ser. No. is 03119895.3, so that the two UEs can receive and transmit P2P signals in the allocated timeslots respectively, and thus P2P communication between two UEs can be implemented.
However, when P2P communication mode is applied in TDD wireless communication systems, if a P2P UE shares the same uplink timeslot as other UEs employing UP-UTRAN-DOWN mode, it may potentially impair uplink synchronization and thus deteriorate the system performance. Specifically, when any UE working in P2P communication mode moves, the distance between the UE and the UTRAN will change, which means the UE establishing uplink synchronization with the UTRAN via random access procedure in cell search procedure after powering on, is likely to lose synchronization with the UTRAN. At this time, corresponding methods are needed to keep the UE in synchronization with the UTRAN. Otherwise, the UTRAN can overhear information on the P2P link between the two UEs during P2P communication process. In this way, if it can't be guaranteed that the P2P signals from the two UEs arrive at the UTRAN at the same time, the spreading code demodulated at the UTRAN won't be orthogonal, thus the system performance is deteriorated.
But P2P communication mode changes the UP-UTRAN-DOWN communication mode in conventional TD-SCDMA systems, i.e.: in conventional TD-SCDMA systems, the UTRAN is involved in any ongoing communication as information source or destination or relayer/forwarder, thus it can monitor and control the UE's uplink signal transmitting time through a specific traffic burst structure, so as to maintain uplink synchronization between the UE and the UTRAN in connect mode. While during P2P communication, the UE has no dedicated traffic channel linked with the UTRAN, so it can only overhear information between two P2P UEs, which means that the UTRAN can't maintain uplink synchronization with conventional mode by adjusting the uplink synchronization TA (time advance) with a specific traffic burst structure even if it can overhear and estimate the communicating UE's synchronization shift information. Therefore, for communication systems like TD-SCDMA systems that have a strict requirement for uplink synchronization, how to maintain uplink synchronization between a moving UE and the UTRAN, will be a great challenge.
An method and apparatus for maintaining uplink synchronization between the moving UE and the UTRAN to be controlled by sending synchronization shift information via the paging control channel in the downlink common control channel between the UE and the UTRAN, is described in the patent application entitled “A Method and Apparatus for Uplink Synchronization Maintenance with P2P Communication in Wireless Communication Networks”, filed by KONINKLIJKE PHILIPS ELECTRONICS N.V. on Mar. 7, 2003, application Ser. No. 03119895.3, and incorporated herein as reference.
In the method and apparatus for uplink synchronization maintenance as described in the application, with no direct dedicated channel between the UTRAN and the UEs, the PCCH (paging control channel) in existing downlink common control channel is utilized to send synchronization shift information to the UEs, thus radio resource is saved. But the method and apparatus brings two new problems: first, when the UTRAN can't overhear information transferred on the P2P direct communication link, it will require the UEs to increase transmission power for P2P signals, which violates the transmission power requirement for P2P communication; second, sending synchronization shift information by using downlink common control channel, will potentially cause overload on downlink common control channel. Detailed description will be given below to the two problems:
First, the method and apparatus for uplink maintenance as described in the application document with application Ser. No. as 03119894.5, estimates the synchronization shift information of two UEs, according to the UTRAN's overheard signal on the direct link between the two P2P UEs. Then, the synchronization shift information obtained through estimation is sent to the two UEs via downlink common control channel between the UTRAN and the UEs. However, when failing to overhear information transferred on P2P direct communication link, the URAN has to send poll message to the UE to increase its power to transmit P2P signals till it can overhear information transferred over P2P direct communication link. This means transmission power of the P2P communicating UEs is expected to transmit signals to each other with transmission power high enough to reach the UTRAN, otherwise, the UTRAN may fail to overhear signals on P2P direct link and therefore fail to maintain uplink synchronization for each UE. On the other side, In accordance with the purpose of P2P communication establishment, P2P link generally is established between two UEs who are very close to each other, and in order to satisfy the same quality for the same services, the P2P communicating UEs are expected to transmit P2P signals to each other with much lower transmission power than that between the UE s and the UTRAN. With lower transmission power, P2P communication can benefit the system with reduced system interference and saved battery consumption, and therefore improve system capacity. As described above, the UE's transmission power may be increased for the UTRAN to overhear signals on the P2P link, which imposes contradicts with increasing the system performance and capacity by using lower transmission.
Second, in accordance with the method and apparatus for uplink synchronization maintenance as described in the application whose application Ser. No. is 03119894.5, when overhearing signals transferred on the direct link between two P2P communicating UEs, the UTRAN can get the synchronization shift information by estimating the midamble in the traffic burst structure. Then, the synchronization shift information is sent to the P2P communicating UEs via PCH on downlink control channel, to adjust the UEs' TA for transmitting P2P signals, and thus the UE and the UTRAN can be kept synchronized. According to TD-SCDMA protocol standard, we know a specific PCH is shared by several UEs and the associated PCH for each UE is Identified by the calculation result of its IMSI (International Mobile Station Identification, only the last three digitals is used in this case), DRX (Discontinuous Reception) cycle and the number of common control channels. Each UE has a different IMSI of its own, while DRX cycle and the number of common control channel are determined by the network system configuration. For each UE sharing the same PCH, all calculation results for identifying each UE's PCH and calculated according to the IMSI, DRX cycle and the number of common control channels, will fall in the same PCH block in paging cycle. If there are many UEs camping in the same cell and some of them happen to work in P2P communication mode and share the same PCH, the synchronization maintenance will avoidably add great overload to the PCH, especially when the UTRAN fails to overhear the ongoing signals on the P2P links in order to keep uplink synchronization of those UEs. The overloaded PCH may potentially block normal paging from outside of the cell and impair the system performance in the mobile communication system.